The photographic production of relief images in a hydrophilic colloid layer is known from the graphic art field where relief images serve as photoresist in the manufacture of graphic art copies or in the production of a printing plate.
Relief images serving as photoresist or mask image are produced in a common way by wash-off processing resulting in a selective removal of unhardened hydrophilic colloid layer portions as described e.g. in European Patent Specification No. 0 036 221.
The relief images produced by wash-off processing have a discontinuous character in that the relief is formed by selective removal of the imaging layer leaving residual relief portions that all have the same thickness.
In some applications it is desirable to have a relief element containing protruding portions of different thickness carry out to pattern-wise control e.g. using electrical and or optical properties.
One of these applications is the control of electro-optical properties of multi-colour liquid crystal display panels by varying the gap-width of the liquid crystal layer as a function of each colour to be displayed. A multi-colour liquid crystal display panel with varying gap-width is represented by cross-sectional view in FIG. 1 of the article "Full-Color Multi-Gap LC-TV Display Panel Addressed by a-Si TFTs" by Sadayoshi Hotta et al., in SID 86 Digest (1986) p. 296-297.
A survey of liquid crystal (LC) materials and device developments is given in IEEE Spectrum, Nov. 1972, p. 25-29, in Electronic Engineering, Aug. 1974, p. 30-37 and in OEP, Feb. 1985 under the title: "Liquid Crystal Gets Second Look As a Promising Display Medium", p. 42-47. From the last mentioned article and the contents of the article "Multiplexed Liquid Crystal Matrix Displays" by J. Duchene in the periodical "Displays, Jan. 1986, p. 3-11 can be learned that the twisted nematic (TN) liquid crystal displays (LCDs) are the most promising candidates for the production of flat panel multi-colour displays comparable in image quality with multi-colour cathode ray tubes.
The operating principles of a TN-liquid crystal display are illustrated in FIG. 10 of IEEE Spectrum, Nov. 1972, p. 29. A cross section of a multi-colour TN-LCD operating with electrode pixels covered with colour filter elements is given in FIG. 2(a) of IEEE Transactions on Electronic Devices, Vol. ED-30, No. 5, May 1983, p. 503.
One of the problems encountered with said TN-LCDs is their optical behaviour in the "off" state wherein their light transmission between parallel polarizers has to be zero or very small to obtain sufficient contrast between the "on" and "off" states of the picture elements (pixels).
As explained in the last mentioned reference and in IEEE Transactions on Electronic Devices, Vol. ED-30, No. 5, May 1983, FIG. 4, the transmittance of a TN-cell with parallel polarizers at the "off" state is a function of .DELTA.n.multidot.d/.lambda., wherein .DELTA.n represents a value for optical anisotropy (birefringence) corresponding with the difference of the refraction indices of the LC along perpendicular optical axes of maximum and minimum refraction, "d" represents the cell gap and .lambda. represents the wavelength.
From said FIG. 4 being reproduced here in the set of accompanying drawings as FIG. 1, one observed that for particular ratios of .DELTA.n.multidot.d/.lambda. the transmittance (% T) is zero, which means that for each different colour wavelength, e.g. red (R), green (G) or blue (B), a different gap-width (d) applies for which there is minimum transmittance. For example, a practically satisfied gap-width condition for green light (wavelength 550 nm) obtained by experiment is 1.4 .mu.m. Taking into account the fact that each display cell is filled with the same LC compound or mixture of LC compounds, the .DELTA.n value for each display cell is a fixed value so that for full light extinction in the already mentioned "off" state, longer wavelengths, e.g. red light, require a broader gap width and smaller wavelengths, e.g. blue light, require a smaller gap-width.
As can be learned from FIG. 2(a) of IEEE Transactions on Electronic Devices, Vol. ED-30, No. 5, May 1983, p. 503 a full-colour image is produced by applying filter elements, red (R), green (G) and blue (B) for each picture element (pixel) in correspondence with energizable electrodes or electrode segments. The liquid crystal arranged in a gap between a pair of electrode elements acts as a light valve in TN-LCDs in such a way that the twisted nematic crystals in the electrically non-energized state of the electrode elements block the light passing through the filter element in correspondence with said elements and, conversely, allow light to pass through a filter element in correspondence with a pair of energized electrode elements.
A method for the production of a multicolour optical filter for use as a faceplate of a television camera tube by chromogenic development of a photographic silver halide emulsion layer is described in U.S. Pat. No. 4,271,246.
Although the present invention is primarily intended for the production of a multi-colour filter relief element suited for application in liquid crystal displays, other applications are possible.
For example, according to an application not necessarily related to multi-colour liquid crystal devices, thin transparent layers having a relief structure can be used for the production of interference patterns that may serve e.g. as hologram or colour display based on the phenomenon of destructive light interference.
Where a transparent relief layer is present on a reflective support, if the thickness of the relief regions of such layer is properly selected relative to the wavelength of monochromatic viewing light or the thickness(s) of one or more regions thereof in relation to the wavelength(s) of one or more spectral regions of polychromatic light, according to well-known optical principles, then advantage can be taken of the effects of destructive interference to create desirable light patterns. For the description of the principles of destructive interference reference is made to the book "University Physics" of Francis Weston Sears and Mark W. Zemansky - Addison - Wesley Publishing Company, Reading Mass., 4th ed., (1972), p. 600-602.